MAX9776 Maxim Integrated Products, MAX9776 Datasheet - Page 22

• Manufacturer Part Number: MAX9776
• Description: (MAX9775 / MAX9776) Stereo Class D Audio Subsystem
• Manufacturer: Maxim Integrated Products

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2 x 1.5W, Stereo Class D Audio Subsystem
with DirectDrive Headphone Amplifier
The MAX9775/MAX9776 use Maxim’s unique, patented
modulation scheme that eliminates the LC filter
required by traditional Class D amplifiers, improving
efficiency, reducing component count, conserving
board space and system cost. Conventional Class D
amplifiers output a 50% duty-cycle square wave when
no signal is present. With no filter, the square wave
appears across the load as a DC voltage, resulting in
finite load current, increasing power consumption,
especially when idling. When no signal is present at the
input of the MAX9775/MAX9776, the outputs switch as
shown in Figure 4. Because the MAX9775/MAX9776
drive the speaker differentially, the two outputs cancel
each other, resulting in no net idle mode voltage across
the speaker, minimizing power consumption.
Traditional single-supply headphone amplifiers have
outputs biased at a nominal DC voltage (typically half
the supply) for maximum dynamic range. Large cou-
pling capacitors are needed to block this DC bias from
the headphone. Without these capacitors, a significant
amount of DC current flows to the headphone, resulting
in unnecessary power dissipation and possible dam-
age to both headphone and headphone amplifier.
Maxim’s patented DirectDrive architecture uses a charge
pump to create an internal negative supply voltage. This
allows the headphone outputs of the MAX9775/MAX9776
to be biased at GND, almost doubling dynamic range
while operating from a single supply. With no DC compo-
nent, there is no need for the large DC-blocking capaci-
tors. Instead of two large (220µF, typ) tantalum
capacitors, the MAX9775/MAX9776 charge pump
requires two small ceramic capacitors, conserving board
space, reducing cost, and improving the frequency
response of the headphone amplifier. See the Output
Power vs. Load Resistance and Charge-Pump Capacitor
Size graph in the Typical Operating Characteristics for
details of the possible capacitor sizes. There is a low DC
voltage on the amplifier outputs due to amplifier offset.
However, the offset of the MAX9775/MAX9776 is typically
1.4mV, which, when combined with a 32Ω load, results in
less than 44nA of DC current flow to the headphones.
22
Filterless Modulation/Common-Mode Idle
______________________________________________________________________________________
DirectDrive
In addition to the cost and size disadvantages of the
DC-blocking capacitors required by conventional head-
phone amplifiers, these capacitors limit the amplifier’s
low-frequency response and can distort the audio sig-
nal. Previous attempts at eliminating the output-cou-
pling capacitors involved biasing the headphone return
(sleeve) to the DC bias voltage of the headphone
amplifiers. This method raises some issues:
1) The sleeve is typically grounded to the chassis.
2) During an ESD strike, the driver’s ESD structures are
3) When using the headphone jack as a lineout to
Figure 4. Outputs with No Input Signal
V OUT+ - V OUT- = 0V
Using the midrail biasing approach, the sleeve must
be isolated from system ground, complicating prod-
uct design.
the only path to system ground. Thus, the amplifier
must be able to withstand the full ESD strike.
other equipment, the bias voltage on the sleeve may
conflict with the ground potential from other equip-
ment, resulting in possible damage to the amplifiers.
V IN = 0V
OUT+
OUT-